Dosing calculation for Ceftriaxone 400 mg IM

[gigi607]

Hi there

I'm a nursing student and I have a question regarding Ceftriaxone dosing calculation. I'm not sure if I should start with the 500mg vial reconstituted with 1.0ml of lidocaine or the 1g vial reconstituted with 2.1 ml for lidocaine. Both the 500 mg and 1g vial will give me 350mg/ml after reconstitution but I don't understand how to get the remaining 50 mg needed for the order. I've tried to figure this out with dimensional analysis and I just confuse myself over and over. I'd really appreciate any sage wisdom that can be offered.

Thank you in advance!

[Thujone]

You want 400 mg, but you only have 350 mg per ml. So, you should create a want-have divsion equation.

It works like this:

Want / Have

so...

400mg / 350mg (per 1ml) = 1.14 ml

If you need to divide the dose due to having to give in into two different thighs, then just divide by two.

1.14 / 2 = .57

so two syringes with .57 that is

Now the tricky part, how do you get the exact amount with such numbers?

The answer: Once you have 1.14 ml you should be able to get fairly close to this with a regular 5 cc syringe. Just get the liquid in between the 1.10 and 1.20 cc mark and you are going to have as close to the exact amount as possible.

So you now have 1.15 ml of Rocephin in 1, 5 cc syringe. This breaks down to .575 if you need to break it in two syringes. Use the same method as above. Get the amount of medicine in each one as close to .6 cc as you can in both syringes. Often, you will have slightly less in one of the syringes, but that is okay.

Also, I personally would pick the 500 mg bottle. Picking the 1 G bottle does nothing to help solve your delimma of having the exact amount of medication in the syringe. The truth is, when the numbers are like this, you never have the exact amount of medication, but you can get so close to the exact amount that the results in giving the medication at such a small amount off would not make a difference. More often then not, the practitioner is prescribing the medicine on a formula of 50 to 75 mg per kilogram (for Rocephin in pediatric patients), so the prescription it self is at the providers discretion, so if the patient was 8kg then the effective dose could be from 400 to 600, so having such a small defenciancy or just slightly over, and I'm talking lik 0.1 ml over and under would not matter at all.

[Mavrick, BSN, RN]

[TABLE]

[TR=class: First]

[TH=align: center]Vial Dosage Size[/TH]

[TH=colspan: 2, align: center] Amount of Diluent to be Added

[/TH]

[/TR]

[TR=class: Last]

[TH=align: center][/TH]

[TH=align: center] 250 mg/mL

[/TH]

[TH=align: center]350 mg/mL

[/TH]

[/TR]

[TR=class: First]

[TD=align: center]500 mg[/TD]

[TD=align: center] 1.8 mL

[/TD]

[TD=align: center]1.0 mL[/TD]

[/TR]

[TR=class: Last]

[TD=align: center]1 gm[/TD]

[TD=align: center] 3.6 mL

[/TD]

[TD=align: center]2.1 mL

[/TD]

[/TR]

[/TABLE]

This is what the package insert says and I too am confused about the final concentration. Can anyone who actually works with this stuff clarify?

[KelRN215, BSN, RN]

[TABLE]

[TR=class: First]

[TH=align: center]Vial Dosage Size[/TH]

[TH=colspan: 2, align: center] Amount of Diluent to be Added[/TH]

[/TR]

[TR=class: Last]

[TH=align: center][/TH]

[TH=align: center] 250 mg/mL[/TH]

[TH=align: center]350 mg/mL[/TH]

[/TR]

[TR=class: First]

[TD=align: center]500 mg[/TD]

[TD=align: center] 1.8 mL[/TD]

[TD=align: center]1.0 mL[/TD]

[/TR]

[TR=class: Last]

[TD=align: center]1 gm[/TD]

[TD=align: center] 3.6 mL[/TD]

[TD=align: center]2.1 mL[/TD]

[/TR]

[/TABLE]

This is what the package insert says and I too am confused about the final concentration. Can anyone who actually works with this stuff clarify?

The powdered medication has what's known as a "displacement value." The powdered medication adds to the total volume. You add 1.8 mL of diluent to the vial and your final volume ends up being 2 mL, giving you a final concentration of 250 mg/mL.

[ICareAlot]

Has anyone else ever considered the fact that one hardly ever gets the listed amount of the total diluted volume? For example, for 1G diluted with 2.1 ml you should get a 2.8 ml volume total, but more often than not, one ends up with around 2.6 ml total diluted volume. I've yet to see one person get every particle of this medication fully reconstituted. There is always a small amount of it scattered throughout the bottle, stuck to the sides that just won't come off no matter what you do.

My real question is that does truly affect the strength of the diluted medication? If it does then you can simply do an equation like the following.

750 / X = 1000 / 2.6 ML

This gives you a total of 1.95 ML.

If you just do the want-have equation you get the following:

750 / 350 = 2.14 ML.

As you can see, this creates a marked difference, 2.14 vs 1.95 ML.

I'm not 100% sure what to do about this difference though. What do y'all think?